When a big asteroid zipped past Earth a few months ago, astronomers discovered a companion tagging along with it like a puppy: a small moon. Radar images showed it as a bright sliver spinning around the darker asteroid.

The discovery isn’t all that surprising, though. About 150 asteroids are known to have moons, including a few that have two moons. And some estimates say that about one of every six large asteroids should have a moon.

The first asteroid moon was discovered 20 years ago today. It was found as the Galileo spacecraft flew close to Ida, a large asteroid that orbits between Mars and Jupiter. Galileo’s pictures revealed a moon that was less than a mile long, compared to a length of about 35 miles for Ida. The moon was named Dactyl, for a group of monsters associated with Ida in Greek mythology.

An asteroid moon may form when its parent asteroid is hit by another asteroid, sending a chip flying off into space. The collision has to be at the right speed and angle, though. If it’s too powerful, the chip careens off into space and leaves the asteroid behind.

Some asteroids have not moons, but binary companions — the two objects are about the same size, so they travel through space as a more-or-less equal pair. And some other asteroids are contact binaries — they look like separate rocks that are touching each other.

All of these discoveries show that when you find a good-sized asteroid, there’s a pretty good chance that it’s not alone.

Script by Damond Benningfield, Copyright 2013

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